Cubic carbon would be very hard, it can be made at all. It could hold an incredibly sharp edge. The blade of a Katana is usually made of folded carbon steel, but with carbon fibre and graphene string to make and wrap the handle and guards, composites of carbon nanotubes, graphene and cubic carbon for the blade, and a cubic carbon edge, with maybe a few diamonds and graphite patterning to decorate Carbon Girl’s Katana, a pure carbon Katana could be made for both her and Carbon Man. No other materials are needed at all.

Perfect. But if they aren’t allowed to take them with them for some reason, they can quickly fabricate temporary but lethal substitute using nothing more than carbon tape.